In recent years, as represented by a vehicle-mounted secondary battery mounted on a vehicle as a power supply in a hybrid car which uses an engine and an electric motor in combination and an electric vehicle, the use of a power supply system in which a high voltage is output by connecting a large number of secondary batteries in series is growing. Such a power supply system is configured by connecting, in series, a large number of the secondary batteries of, e.g., 80 cells to 500 cells. Hence, it is difficult to secure the reliability of the entire power supply system.
Specifically, in such a power supply system, variations in characteristics occurring in the manufacturing process of the secondary battery constituting the power supply system cause variations in the accumulated charge amount (accumulated electric quantity) charged in the respective secondary batteries.
When the charge and discharge of the power supply system are repeated in the state where there are variations in the accumulated charge amount, the degradation of the secondary battery larger or smaller in accumulated charge amount than the other secondary batteries may be accelerated. That is, when the power supply system is charged, the secondary battery which is already larger in accumulated charge amount than the other secondary batteries before starting the charge is fully charged earlier than the other secondary batteries. Hence, the secondary battery tends to be overcharged, and the degradation of the secondary battery larger in accumulated charge amount may be accelerated. On the other hand, when the power supply system is discharged, the accumulated charge amount of the secondary battery which is already smaller in accumulated charge amount than the other secondary batteries before starting the discharge is reduced to zero earlier than the accumulated charge amounts of the other secondary batteries. Hence, the secondary battery tends to be overdischarged, and the degradation of the secondary battery smaller in accumulated charge amount may be accelerated.
In the secondary battery of which the degradation is accelerated, its capacity is reduced and its life is shortened. In the case of the power supply system, when a part of the secondary batteries is degraded, the entire power supply system becomes unusable or the reliability thereof is lowered. Therefore, the degradation of the secondary battery caused by the variations in the accumulated charge amount has a large impact on the entire power supply system. Consequently, in the power supply system which uses a large number of the secondary batteries, it is desired to equalize the accumulated charge amounts of the respective secondary batteries, i.e., the terminal voltages thereof.
To cope with this, there is known an equalization circuit which performs equalization processing which reduces imbalance in the accumulated charge amounts and the terminal voltages of the respective secondary batteries (see, e.g., Patent Document 1 and Patent Document 2). In this equalization circuit, the secondary battery which is larger in the accumulated charge amount than the other secondary batteries and is accordingly higher in the terminal voltage than the other secondary batteries is discharged via a resistance load or a transistor. Accordingly, the redundant accumulated charge amount is released by converting the redundant accumulated charge amount into thermal energy.
However, when the redundant accumulated charge is released by converting it into the thermal energy, the ambient temperature of the equalization circuit rises. As a result, the ambient temperature of an electronic component disposed in the vicinity of the equalization circuit may exceed the guaranteed operating range of the electronic component. In order to maintain the ambient temperature within the guaranteed operating range of the electronic component in the vicinity thereof, it is only necessary to set the value of a discharge current when the equalization is performed to a small value. However, when the discharge current is reduced, there has been an inconvenience that a time period required for the equalization processing for reducing the imbalance between the secondary batteries is increased.
Patent Document 1: Japanese Patent Application Laid-open No. 2010-142039
Patent Document 2: Japanese Patent Application Laid-open No. 2008-54416